Austempering/marquenching powder metal parts

ABSTRACT

A powder metal part is made by compaction at room temperature or an elevated temperature followed by sintering, a secondary densification, heat treating, and optional secondary operations. The particulate materials preferably include iron, 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, and 0-1.5 wt % silicon. At least one secondary densification is applied to the part after compaction and pre-sinter/sinters steps to achieve medium to high density. The secondary densification is part of a double-press double-sinter (DPDS) or is a mechanical working depending on the application requirements. The powder metal is heat treated by austempering or marquenching followed by tempering. A unique composite microstructure is achieved from austempering by controlling the powder chemistry and the holding time at an elevated temperature. The combination of a secondary densification and austempering or marquenching produces a high performance powder metal part for demanding applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to the field of powder metallurgy. More particularly, the invention pertains to the formation of a high-performance powder metal part using a manufacturing process including a secondary densification and austempering or marquenching as a heat treatment.

2. Description of Related Art

Powder metallurgy manufacturing processes have been successfully used to produce powder metal parts while providing various advantages. Such powder metal parts can be made into a variety of complex shapes with minimal steps, and with little or no finishing machining. Powder metallurgy also enables a high-volume of metal parts to be produced in an economic fashion. Powder metallurgy processes have been found to be more energy efficient than other processes involving forging and outright machining.

Secondary densifications of powder metals followed by heat treatment are known in the art.

U.S. Pat. No. 6,338,747, issued Jan. 15, 2002 to Kosco, discloses compacting a powder metal composition, sintering the compact, mechanically working the sintered compact, and reheating the part to increase the surface hardness.

U.S. Pat. No. 6,013,225, issued Jan. 11, 2000 to Cadle et al., discloses surface heating followed by re-pressing to increase the surface density of a powder metal part. The part may be re-sintered or heat treated after re-pressing.

PCT Publication No. WO 97/45219, published Dec. 4, 1997 to Shivanath et al., discloses sintering a powder metal composition, rolling the surface, heating the part, and carburizing the part in a vacuum furnace.

Powder metal parts may be used in a variety of applications, but have found only limited use in high-performance applications, such as within an automobile transmission, due to the fact that many powder metal parts suffer from reduced bending fatigue strength and lowered wear resistance.

Austempering and marquenching are special heat treatments, which improve part strength by changing the microstructure of the part.

There is a need in the art for a process of making a powder metal part for a high-performance application, such as an automotive transmission application, whereby the powder metal part exhibits a high surface durability, a high wear resistance, and high rolling contact fatigue.

SUMMARY OF THE INVENTION

A powder metal part is made by compaction at room temperature or an elevated temperature followed by sintering, a secondary densification, heat treating, and optional secondary operations. The particulate materials preferably include iron, 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, and 0-1.5 wt % silicon. At least one secondary densification is applied to the part after compaction and pre-sinter/sinters steps to achieve medium to high density. The secondary densification is part of a double press double sinter (DPDS) or is a mechanical working depending on the application requirements. The powder metal is heat treated by austempering or marquenching followed by tempering. A unique composite microstructure is achieved from austempering by controlling the powder chemistry and the holding time at an elevated temperature. The combination of a secondary densification and austempering or marquenching produces a high performance powder metal part for demanding applications.

A method of producing a powder metal part includes the steps of performing a primary densification on a powder metal composition, sintering the powder metal composition, performing a secondary densification on the powder metal composition, and heat treating the powder metal composition including austempering or marquenching. The powder metal composition preferably includes iron, 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, 0-1.5 wt % manganese, and 0-1.5 wt % silicon. Compacting the composition is preferably done at a pressure of 25 to 65 TSI to result in a green density of 6.4 to 7.4 grams per cubic centimeter (g/cc). The sintering step preferably includes heating the composition to 1200° F. to 1800° F. or 2000° F. to 2400° F. for 15 to 90 minutes.

In an embodiment of the present invention, the steps of performing a primary densification, performing a sinter, and performing a secondary densification are part of a double-press double-sinter operation. The compacting step is preferably done at a pressure of 30 to 65 TSI to a green density of 6.4 to 7.4 g/cc. The sintering step includes heating to 1200° F. to 1800° F. for 15 to 90 minutes. The secondary densification step includes re-pressing the composition at 30 to 60 TSI and high-temperature sintering the composition at 2100° F. to 2400° F. for 20 to 60 minutes.

In another embodiment of the present invention, the secondary densification is mechanical working. The mechanical working is preferably extrusion, swaging, roll burnishing, rolling, or shot peening.

In yet another embodiment of the present invention, the heat treatment is austempering. Austempering preferably includes heating the composition to a temperature of 1500° F. to 2000° F. for 20 to 150 minutes, cooling the composition at a rate of 150° F. to 250° F. per minute to a temperature of 450° F. to 800° F., and holding the composition at a temperature of 450° F. to 800° F. for 20 to 120 minutes. The step of holding is preferably done in a molten salt medium or in a gas furnace. The holding temperature is preferably slightly above the martensite starting temperature of the composition such that a phase transformation to a bainite microstructure occurs.

In another embodiment of the present invention, the heat treatment is marquenching. Marquenching preferably includes austenitizing the composition at a temperature of 1500° F. to 2000° F. for 20 to 90 minutes, quenching the composition to a uniform temperature slightly above or below the martensite starting temperature of the composition, and cooling the composition at a rate of 40° F./min to 150° F./min to prevent a drastic temperature gradient in the composition. The step of cooling is preferably done in a hot oil medium or in a gas furnace. Marquenching preferably includes tempering the composition at a temperature of 300° F. to 1000° F. for 20 to 90 minutes.

In yet another embodiment of the present invention, the method further includes performing at least one secondary operation on the composition after the step of performing a heat treatment. The secondary operation may be honing, broching, deburring, drilling, or turning.

The powder metal part made by the method is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a method of manufacturing a high-performance powder metal part in a first embodiment of the present invention.

FIG. 2 shows a method of manufacturing a high-performance powder metal part in a second embodiment of the present invention.

FIG. 3 shows a method of manufacturing a high-performance powder metal part including austempering or marquenching in an embodiment of the present invention.

FIG. 4 shows a method of manufacturing a high-performance powder metal part including austempering in an embodiment of the present invention.

FIG. 5 shows a method of manufacturing a high-performance powder metal part including marquenching in an embodiment of the present invention.

FIG. 6 shows a method of manufacturing a high-performance powder metal part including DPDS in an embodiment of the present invention.

FIG. 7 shows a method of manufacturing a high-performance powder metal part including mechanical working in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Austempering is an isothermal heat treatment which is used to produce a bainite microstructure in ferrous materials. The ferrous material is heated to an austentizing temperature then quickly cooled by quenching in a bath to a quench temperature just above the martensite starting temperature (M_(S)) for the material. The ferrous material is held in the bath to allow isothermal transformation to bainite. The material is then cooled to room temperature. The amount of bainite in the material after austempering depends on the length of time the material is held in the bath. At the start of bainitic transformation (B_(s)), the material has 0% bainite. If the material is held isothermally until the finish of bainitic transformation (B_(f)) is reached, the material has 100% bainite. Cooling the material at a time between the B_(s) and the B_(f) time points produces a material with a percentage of bainite between 0 and 100%. Austempering increases the ductility, toughness, and strength of the material at a given hardness. Austempering also decreases cracking and distortion quenching of the material.

Marquenching, also known as martempering, is a heat treatment that produces primarily martensite that is untempered and brittle. The ferrous material is heated to an austentizing temperature and then quenched in a bath to a quench temperature just above or just below the martensite starting temperature (M_(S)). The material is then isothermally held in the bath until the temperature is uniform throughout the material, usually for a time less than the time it takes to reach the start of banitic transformation (B_(s)). The material is then cooled relatively slowly to room temperature to avoid drastic temperature gradients in the material. Marquenching minimizes distortion, cracking, and residual stress in the material.

A double-press double-sinter (DPDS) process includes the steps of compacting, pre-sintering, re-pressing, and high temperature sintering of a material.

A powder metal part of the present invention preferably includes 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, 0-1.5 wt % manganese, and 0-1.5 wt % silicon. Iron preferably makes up the balance of the powder metal composition, although those skilled in the art will recognize that impurities may also be found in the mixture. Common impurities in a powder composition include, but are not limited to, 0.08-0.15 wt % oxygen and trace amounts of sulfur and phosphorus.

A method of the present invention includes the step of performing a secondary densification and the step of performing a heat treatment of austempering or marquenching to produce a medium to high density and high performance powder metal part.

Depending upon performance requirements for the powder metal part, mechanical working or the second press of DPDS may be used as the secondary densification in manufacturing the part. For DPDS, the metallurgical powder is compacted at a pressure of 30 to 65 tons per square inch (TSI) to a green density of 6.4 to 7.4 g/cc. Compaction may be done at room temperature or an elevated temperature up to 450° F. Pre-sinter is then conducted at 1200° F. to 1800° F. for 15 to 90 minutes. Pre-sintering is followed by re-pressing at 30 to 60 TSI. Finally, the part is high-temperature sintered at 2100° F. to 2400° F. for 20 to 60 minutes.

If high surface durability and high precision are needed, mechanical working is applied to increase the portion density and improve the dimensional accuracy of the part. Mechanical working processes include, but are not limited to, extrusion, swaging, roll burnishing, rolling, and shot peening. Mechanical working preferably increases the portion density of the worked portion of the part to greater than 7.4 g/cc. Mechanical working preferably occurs after compaction and sintering or annealing or tempering. The sinter step prior to mechanical working may be a regular sinter (2000° F. to 2100° F. for 20 to 60 minutes) or a high-temperature sinter (2100° F. to 2400° F. for 20 to 60 minutes) depending on the powder composition and the application requirements. For medium- to high-carbon steel, the part is preferably annealed or tempered to improve the formability of the part prior to mechanical working. An annealing/tempering cycle preferably includes holding the part at a temperature of 500° F. to 1800° F. for 15 to 90 minutes and then cooling the part at a rate of less than 70° F. per minute. Low-carbon steel typically does not require annealing and tempering.

After DPDS or after compaction, sintering, and mechanical working, the part is heat-treated by austempering or marquenching. When a bainite microstructure or a composite bainite/martensite structure is desired, a heat treatment by austempering is used. When a martensite structure is desired, a heat treatment by marquenching is used. Austempering preferably includes the following steps. The part is heated to a temperature of 1500° F. to 2000° F. for 20 to 150 minutes. The part is then cooled at a rate of 150° F. to 250° F. per minute to a temperature of 450° F. to 800° F. Then, the part is held at a temperature of 450° F. to 800° F. for 20 to 120 minutes to allow for a phase transformation. The holding temperature is slightly above the martensite starting temperature (M_(S)) of the part, which depends on the composition of the part. The structure is converted to virtually 100% bainite or to a mixture of bainite and martensite by varying the holding time. The percentage of martensite depends on the chemical composition of the part, the holding time, and the cooling rate. If martensite is present in a significant amount in the austempered part, an additional step of tempering is preferred to relieve stress in the part and increase its toughness. Austempering increases the ductility, toughness, and strength of the part at a given hardness and reduces distortion of the part.

Marquenching may be done in place of austempering, and preferably includes the following steps. The part is austenitized at a temperature of 1500° F. to 2000° F. for 20 to 90 minutes. The part is then quenched at a temperature slightly above or below its M_(S) (350° F. to 600° F.) until the temperature is uniform throughout the part. The time to reach a uniform temperature depends on the dimensions of the part. For automotive transmission parts, the part is quenched for 2 to 30 minutes. The part is cooled at a moderate rate to prevent drastic temperature gradients in the part. The cooling rate is preferably in the range of 40° F./min to 150° F./min. The microstructure after marquenching is typically primarily martensite. A subsequent step of tempering at a temperature of 300° F. to 1000° F. for 20 to 90 minutes is preferred. Marquenching minimizes cracking, residual stresses, and distortion in the powder metal part.

A method of the present invention produces a part of the present invention having a high surface durability, high wear resistance, high rolling contact fatigue, and enhanced heat treating properties.

FIG. 1 shows a first embodiment of a method of manufacturing a high-performance powder metal part. A powder metal composition is compacted (20) at a desired pressure to a desired green density. A preferred pressure range is 25 to 65 TSI, and a preferred green density range is 6.4 to 7.4 g/cc. The compaction occurs in the desired shape, or substantially in the desired shape for the final part to be produced. Many powder metal compositions may be used in conjunction with the embodied methods, and their equivalents, disclosed herein. One such preferred powder metal composition has a chemical composition as listed in Table 1. It should be readily understood that other powder metal compositions may be used with the embodied methodologies disclosed herein. TABLE 1 Element Percentage by Weight (wt %) Cu 0-2.0 wt % C 0.15-0.9 wt % Mo 0.5-2.0 wt % Ni 0.5-4.5 wt % Cr 0-4.0 wt % Mn 0-1.5 wt % Si 0-1.5 wt % Fe balance

After the powder metal composition is compacted (20), the green compact is sintered (22) at a desired temperature for a desired amount of time. A preferred sintering atmosphere is nitrogen-based, typically 90% nitrogen and 10% hydrogen. These sinter atmospheric conditions reduce surface oxidation and help control the carbon content of the powder metal part. A preferred sinter temperature range is 2000° F. to 2400° F., except for the case of the first sinter of a DPDS, where the preferred temperature range is 1200° F. to 1800° F. A preferred sinter time is 15 to 90 minutes. After the compact is sintered (22), a secondary densification (24) is performed. Preferred secondary densification processes include re-pressing in a double press double sinter or mechanically working. Finally, the powder metal part is heat treated (26) to produce a bainite or martensite structure. Preferred heat treatments include austempering and marquenching. A secondary operation may or may not be desired (27) for the part following the heat treatment. Secondary operations (28) include but are not limited to honing, broching, deburring, drilling, and turning. The end result is a high-performance powder metal part (30). A high-performance part of the present invention may have a high overall density of greater than 7.3 g/cc or a high surface density of greater than 7.4 g/cc to give it high strength and high durability.

FIG. 2 shows a second embodiment of a method of manufacturing a high-performance powder metal part. First, the powder metal composition is compacted (32) at a desired pressure to a desired green density. The next steps depend on whether or not a high surface durability, high rolling contact fatigue, or high precision is required (34) in the final product. If at least one of these features is required and the metal composition has a high carbon percentage (35), the composition is sintered and annealed/tempered (36), cooled to room temperature (38), and mechanically worked (40). If the powder composition is a low-carbon composition, the substeps of sintering and annealing/tempering (36) and cooling (38) are not necessary and the compact may be directly mechanically worked (40). The part may also be directly worked after sintering to increase the surface density. If none of the final product features is required, the composition is pre-sintered (42), re-pressed (44), and high-temperature sintered (46) to complete a double-press double-sinter operation.

After the secondary densification process, depending on whether or not a bainite microstructure is desired in the part (47), the composition is either austempered (48) or marquenched (50) and tempered (52), followed by an optional secondary operation (54) depending on whether or not a secondary operation is desired (53) to produce the high-performance powder metal part (56).

FIG. 3 shows another embodiment of a method of manufacturing a high-performance powder metal part. In particular, the heat treatment step (26) of FIG. 1 has been replaced by two alternative paths. As before, a powder metal composition is compacted (20) at a desired pressure to a desired green density. After the powder metal composition is compacted (20), the green compact is sintered (22) at a desired temperature for a desired amount of time. After the compact is sintered (22), a secondary densification (24) is performed. Depending on whether or not a bainite microstructure is desired in the part (59), the composition is then heat-treated either by austempering (60) or marquenching (62) followed by tempering (64) to produce a martensite, bainite, or bainite and martensite structure. Again, the heat treatment may be followed by a secondary operation (65) as desired (63). The end result is a high-performance powder metal part (66).

FIG. 4 shows another embodiment of a method of manufacturing a high-performance powder metal part. As before, a powder metal composition is compacted (20) at a desired pressure to a desired green density. After the powder metal composition is compacted (20), the green compact is sintered (22) at a desired temperature for a desired amount of time. After the compact is sintered (22), a secondary densification (24) is performed. The composition is then austenitized at a temperature of 1500° F. to 2000° F. for approximately 20 to 150 minutes (70). The composition is then cooled to a temperature slightly above the M_(S) of the composition (72). This temperature is approximately in the range of 450° F. to 800° F. The cooling rate is preferably 150° F. to 250° F. per minute to reach this temperature. This temperature is then held for approximately 20 to 120 minutes (74) to allow a phase transformation. The composition is then cooled to room temperature (76). The cooling may be followed by a secondary operation (77) as desired (75). The end result is a high-performance powder metal part (78).

FIG. 5 shows yet another embodiment of a method of manufacturing a high-performance powder metal part. As before, a powder metal composition is compacted (20) at a desired pressure to a desired green density. After the powder metal composition is compacted (20), the green compact is sintered (22) at a desired temperature for a desired amount of time. After the compact is sintered (22), a secondary densification (24) is performed. The composition is then austenitized at a temperature of 1500° F. to 2000° F. for approximately 20 to 90 minutes (80). The composition is then quenched at a temperature near the M_(S) of the composition (82). This temperature is approximately in the range of 350° F. to 600° F. This temperature is then held until the temperature is substantially uniform throughout the composition (84) for a holding time of approximately 2 to 30 minutes depending on the dimensions of the part. The composition is then cooled at a moderate rate of approximately 40° F./min to 150° F./min to room temperature (86). The part is then tempered at 300° F. to 1000° F. for approximately 20 to 90 minutes (88) to relieve stress and increase toughness. The tempering (88) may be followed by a secondary operation (not shown). The end result is a high-performance powder metal part (90).

FIG. 6 shows another embodiment of a method of manufacturing a high-performance powder metal part. A powder metal composition is compacted at 30 to 65 TSI to achieve a green density of 6.4 to 7.4 g/cc (100). Then, the composition is pre-sintered at 1200° F. to 1800° F. for approximately 15 to 90 minutes (102). The composition is then re-pressed at 30 to 60 TSI (104). Finally, a high-temperature sintering at 2100° F. to 2400° F. for 20 to 60 minutes (106) completes the double-press double-sinter steps. Depending on whether or not bainite is desired in the powder metal part (107), the composition is then heat-treated either by austempering (108) or marquenching (110) followed by tempering (112) to produce a martensite, bainite, or bainite and martensite structure. Again, the heat treatment may be followed by a secondary operation (113) as desired (111). The end result is a high-performance powder metal part (114).

FIG. 7 shows yet another embodiment of a method of manufacturing a high-performance powder metal part. A powder metal composition is compacted at 25 to 65 TSI to achieve a green density of 6.4 to 7.4 g/cc (120). Then, the composition is sintered at 2000° F. to 2400° F. for approximately 20 to 60 minutes (122). Depending on the level of carbon in the composition (123), the part may be annealed/tempered (124) prior to mechanical working (126). For medium to high carbon steel, the composition is annealed/tempered (124) to improve the formability of the parts before mechanical working. Annealing/tempering (124) is not necessary for low carbon steel. An annealing/tempering cycle includes holding the composition at a temperature of 500° F. to 1800° F. for 15 to 90 minutes and then cooling the composition at a rate of less than 70° F. per minute. The composition then undergoes a secondary densification by mechanically working it (126) to increase the portion density to greater than 7.4 g/cc. Depending on whether or not a bainite microstructure is desired (127) in the part, the composition is then heat-treated either by austempering (128) or marquenching (130) followed by tempering (132) to produce a martensite, bainite, or bainite and martensite structure. Again, the heat treatment may be followed by a secondary operation (133) as desired (131). The end result is a high-performance powder metal part (134).

Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention. 

1. A method of producing a powder metal part comprising the steps of: a) performing a primary densification on a powder metal composition; b) sintering on the powder metal composition; c) performing a secondary densification on the powder metal composition; and d) heat treating the powder metal composition comprising austempering or marquenching the powder metal composition.
 2. The method of claim 1, wherein the powder metal composition comprises iron, 0-2.0 wt % copper, 0.15-0.9 wt % carbon, 0.5-2.0 wt % molybdenum, 0.5-4.5 wt % nickel, 0-4.0 wt % chromium, 0-1.5 wt % manganese, and 0-1.5 wt % silicon.
 3. The method of claim 1, wherein step a) comprises compacting the composition at a pressure of 25 to 65 TSI to result in a green density of 6.4 to 7.4 g/cc.
 4. The method of claim 1, wherein step b) comprises heating the composition to 1200° F. to 1800° F. or 2000° F. to 2400° F. for 15 to 90 minutes.
 5. The method of claim 1, wherein steps a) through c) comprise a double press double sinter.
 6. The method of claim 5, wherein step a) comprises compacting the composition at a pressure of 30 to 65 TSI to result in a green density of 6.4 to 7.4 g/cc, step b) comprises heating to 1200° F. to 1800° F. for 15 to 90 minutes, and step c) comprises the substeps of: e) re-pressing the powder metal composition at 30 to 60 TSI; and f) high-temperature sintering the powder metal composition at 2100° F. to 2400° F. for 20 to 60 minutes.
 7. The method of claim 1, wherein the secondary densification is a mechanical working.
 8. The method of claim 7, wherein the mechanical working is selected from the group consisting of: a) extrusion; b) swaging; c) roll burnishing; d) rolling; and e) shot peening.
 9. The method of claim 1, wherein the heat treatment is austempering.
 10. The method of claim 9, wherein austempering comprises the substeps of: e) heating the composition to a temperature of 1500° F. to 2000° F. for 20 to 150 minutes; f) cooling the composition at a rate of 150° F. to 250° F. per minute to a temperature of 450° F. to 800° F.; and g) holding the composition at a temperature of 450° F. to 800° F. for 20 to 120 minutes.
 11. The method of claim 10, wherein substep g) is performed in a molten salt medium or in a gas furnace.
 12. The method of claim 10, wherein the temperature in step g) is selected to be slightly above a martensite starting temperature of the composition such that a phase transformation to a bainite microstructure occurs in substep g).
 13. The method of claim 1, wherein the heat treatment is marquenching.
 14. The method of claim 13, wherein marquenching comprises the substeps of: f) austenitizing the composition at a temperature of 1500° F. to 2000° F. for 20 to 90 minutes; g) quenching the composition to a uniform temperature slightly above or below a martensite starting temperature of the composition; and h) cooling the composition at a rate of 40° F./min to 150° F./min to prevent a drastic temperature gradient in the composition.
 15. The method of claim 14, wherein substep g) is performed in a hot oil medium or in a gas furnace.
 16. The method of claim 14 further comprising the step of tempering the composition at a temperature of 300° F. to 1000° F. for 20 to 90 minutes.
 17. The method of claim 1 further comprising the step of performing at least one secondary operation on the composition after the step of performing a heat treatment.
 18. The method of claim 17, wherein the at least one secondary operation is selected from the group consisting of: a) honing; b) broching; c) deburring; d) drilling; and e) turning.
 19. A powder metal part made by the method of claim
 1. 20. The powder metal part of claim 19 further comprising a bainite microstructure and an overall density of greater than 7.3 g/cc or a density of greater than 7.4 g/cc. 